intraseasonal variability in the summer south china sea...

Intraseasonal variability in the summer South China Sea: Wind jet,

cold filament, and recirculations

Shang-Ping Xie,1 Chueh-Hsin Chang,1 Qiang Xie,2 and Dongxiao Wang2

Received 21 March 2007; revised 2 June 2007; accepted 16 July 2007; published 9 October 2007.

[1] A recent study shows that the blockage of the southwest monsoon by the mountainrange on the east coast of Indochina triggers a chain of ocean-atmospheric response,including a wind jet and cold filament in the South China Sea (SCS). We extend thisclimatological analysis by using higher temporal resolution (weekly) to studyintraseasonal variability in summer. Our analysis shows that the development of the windjet and cold filament is not a smooth seasonal process but consists of several intraseasonalevents each year at about 45-day intervals. In a typical intraseasonal event, the windjet intensifies to above 12 m/s, followed in a week by the development of a cold filamentadvected by an offshore jet east of South Vietnam on the boundary of a double gyrecirculation in the ocean. The double gyre circulation itself also strengthens in response tothe intraseasonal wind event via Rossby wave adjustment, reaching the maximumstrength in 2 to 3 weeks. The intraseasonal cold filaments appear to influence thesurface wind, reducing the local wind speed because of the increased static stability in thenear-surface atmosphere. To first order, the above sequence of events may be viewed asthe SCS response to atmospheric intraseasonal wind pulses, which are part of theplanetary-scale boreal summer intraseasonal oscillation characterized by the northeastwardpropagation of atmospheric deep convection. The intraseasonal anomalies of sea surfacetemperature and precipitation are in phase over the SCS, suggesting an oceanicfeedback onto the atmosphere. As wind variations are now being routinely monitored bysatellite, the lags of 1–3 weeks in oceanic response offer useful predictability that may beexploited.

Citation: Xie, S.-P., C.-H. Chang, Q. Xie, and D. Wang (2007), Intraseasonal variability in the summer South China Sea: Wind jet,

cold filament, and recirculations, J. Geophys. Res., 112, C10008, doi:10.1029/2007JC004238.

1. Introduction

[2] The South China Sea (SCS) is located at the south-west corner of the North Pacific Ocean (99�E–121�E,0�N–23�N) with an average depth over 2000 m. The SCSis fenced by mountain ranges to its southwest and northeast(Figure 1). On the southwest side, the Annam Cordillera, acoastal mountain range with an average height of 640 m,stretches 1130 km along the borders of Laos and Vietnamand ends just north of Ho Chi Mihn City (107�E, 11�N). Onthe northeast side, the Cordillera Central extends 270 km onnorthern Luzon Island. The orographic effect of thesemountain ranges significantly affects the SCS atmosphere-ocean coupled system [Xie et al., 2003; Wang, 2004; Xieet al., 2006].[3] The SCS climate is part of the East Asian monsoon

system. The prevailing winds are southwesterly in summer

and northeasterly in winter [Liu and Xie, 1999]. The basin-scale upper ocean circulation of the SCS is mainly driven bythe seasonal monsoon winds [Wyrtki, 1961; Xu et al., 1982;Gan et al., 2006], with significant influence from the LuzonStrait transport [Qu, 2000] and the complex bathymetryincluding the broad Sunda Shelf (<100 m) in the south [Chuet al., 1999; Cai et al., 2002]. In winter, in response to thestrong positive curl of the northeast monsoon, a singlecyclonic gyre occupies the basin embedded with twocyclonic eddies: the West Luzon and East Vietnam eddies[Qu, 2000; Liu et al., 2001]. In summer, the AnnamCordillera, impinged by the southwest monsoon, forces awind jet southeast of Ho ChiMihn City [Xie et al., 2003]. Theresultant wind curl dipole drives a double gyre in the ocean,with a northern cyclonic gyre, a southern anticyclonic gyre,and an eastward jet on the intergyre boundary between 11�Nand 14�N [Shaw and Chao, 1994; Shaw et al., 1999; Kuoet al., 2000; Ho et al., 2000; Fang et al., 2002; Metzger,2003]. A tight anticyclonic recirculation, called the SouthVietnam eddy, appears at the northwest corner of the southerngyre [Xie et al., 2003; Wang et al., 2006]. The annual meansurface mixed layer depth in the SCS is about 40m, with littleseasonal variability throughout most of the basin [Chao et al.,1996; Qu, 2001].

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1International Pacific Research Center and Department of Meteorology,University of Hawaii, Honolulu, Hawaii, USA.

2Laboratory for Tropical Marine Environmental Dynamics, South ChinaSea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou,China.

Copyright 2007 by the American Geophysical Union.0148-0227/07/2007JC004238$09.00

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http://dx.doi.org/10.1029/2007JC004238

[4] The SCS warms up rapidly in spring as part of theplanetary-scale Indo-western Pacific warm pool [Wang andWang, 2006]. In summer, a pronounced basin-scale coolingis observed in the SCS and was traditionally attributed toevaporative cooling by the southwest monsoon [Qu, 2000].High-resolution satellite observations of sea surfacetemperature (SST) reveal coastal upwelling along SouthVietnam and a cold filament that stretches eastward at about12�N from the coast superimposed on the basin-scalecooling [Kuo et al., 2000; Ho et al., 2000]. The orographic-induced southwest wind jet is key to the cold filament’soffshore development, by inducing coastal upwelling anddriving the eastward ocean jet that advects the cold coastalwater offshore [Xie et al., 2003].[5] In a climatological analysis of multiyear satellite

observations, the cold filament begins to develop in June,strengthens and reaches the maximum intensity in July–August, and disappears in September [Xie et al., 2003]. TheAsian summer monsoon, on the other hand, is known todisplay large intraseasonal oscillations [Wang, 2006; Lauand Waliser, 2005]. Strong intraseasonal variability in windand SST has been reported in the Bay of Bengal [Senguptaet al., 2001; Vecchi and Harrison, 2002], the West Arabian

Sea [Vecchi et al., 2004], and the SCS [Gao and Zhou,2002]. The Gao and Zhou study uses coarse-resolution datathat do not resolve important features such as the coldfilament and ocean advective effects in general.[6] The present study examines intraseasonal variability

in the summer SCS using a suite of new satellite datasets ofhigh resolution in space and time. We show that the coldfilament does not simply persist through the summer butinstead experiences 2–3 cycles of development and decay.This intraseasonal variability in SST may simply be aresponse to intraseasonal wind variability or alternativelydue to internal variability of the double gyre circulation. Inmodels, nonlinear interaction of the western boundarycurrents, recirculations and potential vorticity advection isknown to cause chaotic variability of the intergyre eastwardjet (see Dijkstra and Ghil [2005] for a review), which maythen cause the cold filament to fluctuate. To investigatethese possibilities, our study will examine variability inwind and sea surface height (SSH) as well as their relation-ship to the intraseasonal development of the cold filament.Our analysis supports the wind-forced hypothesis.[7] The organization of the paper is as follows. Section 2

describes the data sets. Section 3 investigates oceanic

Figure 1. A summer intraseasonal event in 1999 from 21 July to 11 August. The evolution of the coldfilament (SST in �C, color shading), and surface wind speed (ms�1, contour). The gray shading indicatesthe land orography greater than 250 m. The vector indicates surface wind greater than 4 m/s. Local windspeed minimum over the cold filament is marked with a white ‘‘L.’’

C10008 XIE ET AL.: SOUTH CHINA SEA INTRASEASONAL MODE

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variability by a case study, variance, and composite analyses.Section 4 explores the origin of intraseasonal wind variabilityover the SCS using a lag regression analysis. Section 5examines the feedback of cold filaments on the atmosphericboundary layer. Section 6 summarizes the findings anddiscusses their implications.

2. Data

[8] We use weekly mean data to suppress synopticdisturbances. The primary data sets include SST from asatellite microwave radiometer, surface wind from a satellitescatterometer, and SSH anomalies from satellite altimeters.[9] Launched in November 1997, the Tropical Rainfall

Measuring Mission (TRMM) Microwave Imager (TMI) is apassive microwave sensor with a suite of channels rangingfrom 10.7 to 85 GHz that measures microwave energyemitted by the Earth and its atmosphere to quantifyimportant atmospheric variables, such as water vapor, cloudwater, and rainfall intensity. The addition of a low-frequencychannel at 10.7 GHz (almost cloud transparent) enablesthe TMI to measure the SST in all weather conditionsexcept under heavy rain. The TMI provides completecoverage of the global tropics (38�S–38�N) every threedays. The accuracy of the TMI SST estimate in rain-freeconditions is approximately 0.6�C [Wentz et al., 2000]. Inthis study, the weekly 0.25� � 0.25� gridded TMI SSTdata (version 3a) are used from 10 December 1997 to29 December 2004.[10] A scatterometer measures the backscatter from short

water waves (i.e., capillary and ultragravity waves), whichrespond quickly to changes in winds, to derive near-surfacewind speeds and directions. Since July 1999, the SeaWindsscatterometer on the QuikSCAT satellite has been providingglobal wind measurements with high spatial and temporalresolution under almost all weather conditions except forheavy rain. The SeaWinds scatterometer wind retrievals areaccurate to better than 2 m/s in speed and 20� in direction,accuracies comparable to in-situ point measurements frombuoys [Chelton et al., 2004]. We use the weekly averagedQuikSCAT data on a 0.25� � 0.25� grid from 21 July 1999to 29 December 2004.[11] Satellite altimetric measurements of sea surface

topography contain large geoid undulations, which are notwell determined. One can deduce deviations from theabsolute SSH and study temporal variations of the oceancirculation. We use the weekly averaged merged SSHanomalies (SSHA) from the TOPEX/Poseidon (T/P), Jason,ERS-1, ERS-2, and ENVISAT satellites on a 0.33� lon �0.18� lat grid from 14 October 1992 to 29 December 2004.The merged SSHA better resolves spatial and temporalscales of the ocean circulation, especially the mesoscale,than using one single altimeter [Ducet et al., 2000].Launched on 10 August 1992, the T/P altimeter achievedan unprecedented accuracy of about 4 cm. Much smallerSSH signals (�1 cm) of large spatial scales may be detectedwith random measurement errors filtered out. We obtain theabsolute SSH by adding a recent 10-year mean SSH field tothe merged T/P SSHA data. The time-mean SSH is com-puted using near-surface velocity derived from surfacedrifter data and satellite altimeter observations [Niileret al., 2003].

[12] To map planetary-scale atmospheric patterns, twoadditional data sets are used: the European Center forMedium-Range Weather Forecasting reanalysis (ERA) ofdaily geopotential height and wind at 850 hPa for the periodof 14 October 1992 to 28 August 2002, and the pentad-mean Climate Prediction Center merged analysis ofprecipitation (CMAP) [Xie and Arkin, 1997] for the period14 October 1992 to 20 December 2004. Both data sets areon a 2.5� � 2.5� grid. To be compatible with the primarysatellite data sets, the daily ERA and pentad CMAP data areprocessed into weekly mean values.[13] In the composite and regression analyses, we remove

the seasonal cycle by subtracting the monthly climatologyfrom the weekly mean data (after a temporal interpolation).A 90-day high-pass filter is then applied to isolateintraseasonal variability. Such filters are commonly usedin intraseasonal variability studies and appropriate forvariability over the SCS, which as will be seen, has typicaltimescales of 6–7 weeks.

3. Oceanic Variability

[14] We begin with a case study to introduce the intra-seasonal phenomenon, followed by variance and compositeanalyses to extract its spatiotemporal characteristics.

3.1. Case Study

[15] Figure 1 shows the intraseasonal evolution of theSST and the wind fields from the week of 21 July 1999 tothat of 11 August 1999. It is the first intraseasonal eventafter the QuikSCAT began its science operation. In the thirdweek of July 1999, the orographically induced wind jet(>12 m/s) appears off of Ho Chi Mihn City centered at111�E, 11�N, with large positive wind curls to the north.Cold water (<27.5�C) pumped up via the coastal upwellingmechanism is seen in the coastal region between 11.5�N and12.5�N, north of the wind jet. A long thin arc of cold water(<28�C), the cold filament, stretches offshore from the coastand curves to the southeast as it reaches the open ocean(Figure 1a). The wind jet intensifies in the following weekand remains strong until the week of 4 August. The arcingcold filament continues to strengthen, reaching the maxi-mum intensity with a minimum SST of 26.5�C 1 week afterthe wind jet begins to wane. By the week of 11 August(2 weeks after the maximum wind speed), the wind relaxes(�8 m/s), and the cold filament diffuses, leaving behind abroad pool of relatively cold water (SST < 28�C) over muchof the central SCS (Figure 1d).[16] Figure 2 superimposes SST (color) on the SSH field

(in contour) during the week of 28 July 1999. A pair ofrecirculation eddies are present, with their boundary around12�N. The cold filament is anchored on the South Vietnamcoast and advected offshore, riding on the northern rim ofthe anticyclonic southern recirculation eddy. The distinct arcstructure of the cold filament and its curving to the southsuggest that the water originating from the western bound-ary of the southern gyre does not mix readily with waterfrom the northern gyre presumably because of their distinctpotential vorticity [Wang et al., 2002]. The southern gyrewestern boundary current and its extension advect water oflow potential vorticity from the south while the northerngyre western boundary current advects high potential


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vorticity. The potential vorticity gradients on the gyreboundary discourage cross-gyre mixing.

3.2. Variance Analysis

[17] Figure 3 shows time-latitude sections of wind speed(contours) and SST (color) along 111�E, a longitude cuttingacross cold filaments. From 1999 to 2004, every summerfrom May to September, there are two to four cooling

events. Each one is associated with a strong basin-widewind pulse (>8 m/s) and a temperature drop of at least 1�C–1.5�C. ISV in non-El Nino years of 2000–2002 is similar tothat in 1999 and 2004 (not shown). Note that compared tothose in other years, summer intraseasonal events are weakin 2003 because the seasonal mean southwest monsoonweakens following the 2002 El Nino [Xie et al., 2003]. Inthe summer following the major El Nino event of 1997–1998, the SCS is abnormally warm, and no intraseasonaldevelopment of the cold filament is observed.[18] We have calculated the root mean square (RMS)

variance of surface wind speed, SST, and SSH by using the90-day high-pass filtered data for their respective availableperiods. The variance is accumulated during June to August(JJA) for SST and wind, and June–September (JJAS) forSSH. The double gyre circulation persists through thesummer after the southwest monsoon begins to weaken inAugust. Figure 4 shows the results (color) superimposed ontheir long-term summer means (contours).[19] The maximum summer mean wind (speed > 9 m/s) is

located southeast of Ho Chi Mihn City with its core at(110�E, 10.5�N), a result of orographic blockage by theAnnam mountain range [Xie et al., 2003]. The RMSvariance of wind, however, displays a pattern different fromthe mean jet, with large values (>2.3 m/s) displaced furthereast in the open ocean in 112�E–118�E, 9.5�N–12�N(Figure 4a). Orographic effects on intraseasonal winds arestill visible; the wind variance nearly vanishes lee of theAnnam range, generating larger wind curls north than southof 11�N. Section 4 discusses how large-scale atmosphericpatterns give rise to intraseasonal wind fluctuations over theSCS.[20] In the summer mean, the double gyre circulation

forms in the ocean in response to the wind curl forcing ofthe southwest wind jet [e.g., Wang et al., 2006]. Themaximum SSH variability (>5.6 cm) is located between11�N and 12�N, slightly north of the gyre boundary (11�N;Figure 4b). This variance maximum may reflect internalvariability of the nonlinear double gyre circulation thattends to peak at the gyre boundary [e.g., Taguchi et al.,

Figure 2. Same intraseasonal event as in Figure 1: SST(�C color shading) superimposed on SSH (cm, contour)on 28 July 1999. The vector indicates the geostrophiccurrents associated with the double gyre system (7�–15�N,108�–115�E). The 72 cm contour is in bold, whichconveniently marks the gyre boundary.

Figure 3. Summer intraseasonal pulses seen in time-latitude sections of SST (�C, color shading) andwind speed (ms�1, contour) at 111�E for the summer (May to September) of (a) 1998, (b) 1999, (c) 2003,and (d) 2004.


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2007], or it may simply be a response to the asymmetricwind-curl forcing that is stronger north of 11�N due to theorographic blockage as discussed above. In subsection3.3.2, we will trace the intraseasonal evolution of SSHanomalies to examine these two possibilities.[21] The maximum upwelling cooling is located near

11�N on the South Vietnam coast. The region of largeintraseasonal SST variability (RMS > 0.8�C) is collocatedwith the mean cold filament, both displaying a distinctivenortheastward development from the Vietnam coast(Figure 4c). Further offshore, the curving arc is not presentin the mean SST map, indicating its variability in position.

3.3. Composite Evolution

[22] This subsection tracks the evolution of SCS intra-seasonal variability (ISV) in the ocean and atmosphere byconstructing lagged composite maps based on an ISV indexdefined as wind speed anomalies averaged in the box of112�E–116�E, 9�N–13�N, where wind ISV is high. Theuse of an alternative SST index averaged in its high varianceregion off the Vietnam coast yields similar results. For thecomposite analysis, 10 strong intraseasonal wind eventswith 90-day high-pass filtered speed anomalies greater than

0.6 standard deviation are chosen for the period of June toAugust 1999–2004.3.3.1. SST and Wind[23] Figure 5 shows the composite anomalies of wind

speed (contours) and SST (color). At lag �1 week, windsare calm and SST above the climatology. In the followingweek, the southwest winds intensify rapidly by 2 m/s overmuch of the SCS, with the maximum increase of 3.5 m/s inthe open-ocean just west of Palawan Island. SST begins abasin-wide cooling of �0.3�C in response to this intra-seasonal wind pulse. At lag +1 week, this wind pulseweakens considerably while the SST cooling reaches itsmaximum, with a distinct offshore development of the coldfilament from the Vietnam coast. SST anomalies in the coldfilament are about �1�C in the filament. The arc structure ofthe cold filament, however, is unclear in the compositebecause of position variations from one event to another. Atlag +2, winds relax further, and SST returns to near normal.Overall, an intraseasonal wind event lasts for 2 weeks, andthe cold filament development lags by 1 week.3.3.2. SSH[24] Figure 6 shows the evolution of composite SSH

anomalies when and after the southwest monsoon intensi-

Figure 4. Standard deviation of intraseasonal anomaly (90-day highpass filtered data; shading) versusclimatological mean (contour) in summer (JJA) for 1999–2004: (a) surface wind speed (ms�1);(b) SSH (cm); and (c) SST (�C). In Figure 4a the gray shading indicates the land orography greaterthan 0.25 km, and the vectors are for climatological wind velocity.

Figure 5. Summer intraseasonal event composites of SST (�C, color shading) and wind speed(ms�1; solid (positive) and broken (negative) contours), based on 10 strong wind events (summer ISVwind index � 0.6 std) during six summers (JJA) from 1999 to 2004. Time lags are in week.


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fies. At lag 0, there is a pair of anomalous circulations offthe Vietnam coast, likely remnants of the previousintraseasonal cycle at the phase of weakened southwestmonsoon. They are anticyclonic north and cyclonic south of10.5�N, representing a weakening of the seasonal meandouble gyre circulation. At lag +1 week, the southernnegative SSH anomalies somewhat strengthen while thenorthern anomalous circulation disappears near the coast.Negative SSH anomalies develop to the east around 114�E,11.5�N, likely in response to the strong positive wind curlsat lag 0. At lag 2, these newly formed negative SSHanomalies strengthen to �4 cm and move westward nearthe Vietnam coast. At lag 3, these negative SSH anomaliesweaken slightly and are confined in the western half of theSCS north of 10�N, representing an intensification andsouthward shift of the offshore jet. To the south, weakpositive anomalies of 2 cm begin to appear around 112�E,8.5�N, which can be traced 2� eastward 1 week earlier.[25] The above analysis suggests a Rossby wave adjust-

ment of the SCS to changes in wind forcing. Figure 7illustrates this adjustment with longitude-time sections ofanomalies of wind curl and SSH along 12�N. In response topositive wind curl forcing at lag 0, negative SSH anomaliesbegin to appear in the eastern basin, then strengthen, andpropagate westward. It takes 2–3 weeks for the negativeSSH anomalies to arrive at the Vietnam coast. FromFigure 7, the Rossby wave speed is estimated at 3.7 cm/sat 12�N, equivalent to a 5-week transit time across the basin.This is in rough agreement with modeling studies of Liu etal. [2001] andWang et al. [2003, 2006]. It is notable that theamplitude of the wind stress curl in Figures 6 and 7 is largeat lag 0 but then this decreases rapidly, although the SSHdoes not. This is because the wind field is more noisy at theweekly timescale, whereas the ocean response occurs at alower frequency, so there is less damping of the signal at lagin SSH.[26] The southwest monsoon is strongest at lag 0. This

wind intensification displays a clear blockage effect by theAnnam mountains, with wind anomalies decreasing rapidlyin the lee (Figure 4b). Wind curls are much strongernorth than south of the wind maximum (0.6 � 0.7 N/m3

versus �0.2 � �0.3 N/m3), an asymmetry that induces ameridionally asymmetric response in ocean circulation.Indeed, the SSH anomalies north of 11�N are larger andbetter organized in space (Figure 6).

[27] At both lags 0 and 1, SSH anomalies are such thatthe double gyre circulation is weaker than normal (Figure 6),ruling out the possibility that the cold filament’s eastwarddevelopment is due to the anomalous increase in theoffshore jet and its advection. Instead, the intensificationof the double gyre circulation lags that of the southwestmonsoon by two weeks, in accordance with the Rossbywave adjustment, after the cold filament has begun decay-ing. Thus the intraseasonal development of the cold filamentis due to the increased coastal upwelling and offshoreadvection by the mean eastward jet.

4. Origin of SCS ISV

[28] In this section, we look beyond the SCS and identifylarge-scale atmospheric patterns associated with the intra-

Figure 6. Same as in Figure 5 but for wind stress curl (10�7 Nm�3, color shading) and SSH(cm, +SSHA in solid, and �SSHA in broken contours) for (a) lag 0, (b) lag 1, (c) lag 2, and (d) lag 3.

Figure 7. Time-longitude section of SSH (cm, in colorshading) and wind stress curl (10�7 Nm�3, contour)anomalies along 12�N. Time lags are in week.


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seasonal wind variability in the summer SCS. To increasethe data length, we use ERA geopotential height and windvelocity (1992–2002) at 850 hPa, a pressure level abovemost of mountains in the region except the Tibetan Plateau.The SCS ISV indices based on the ERA and QuikSCATwind products correlate well with each other at 0.84 for theiroverlapping period. The ERA 850 hPa wind velocity,geopotential height, and the CMAP rainfall are regressedon the SCS summer ISV index based on ERA wind at lagintervals of 1 week.[29] Figure 8 shows the regression maps of ERA wind

(arrows) superimposed on CMAP rainfall (color). At lags�2 and �1, anomalous winds are northeasterly against theprevailing southwesterlies. A band of positive rainfallanomalies extends in a slight southeast tilt from India andthe Bay of Bengal, en route from the SCS to the western

equatorial Pacific. From lag �2 to 0, this rain band movesnorthward and a center of positive rainfall anomalies formsover the northern SCS and the northwestern tropical Pacificto the east. This center of positive rainfall anomalies,together with negative rainfall anomalies over the equatorialIndian Ocean and the Maritime Continent, drives a strongintensification of the southwesterlies over the central SCS atlag 0 (Figure 8c), in geostrophic balance with large merid-ional gradients of geopotential height (not shown). Thusanomalies of both wind and geopotential lag behind theserainfall anomalies by 1 week, peaking at lag 0. A strongcyclonic circulation forms centered on the Luzon Strait atlag 0 and persists into lag +1 with reduced intensity. At lag0, positive rainfall anomalies dissipate, replaced by a titledband of negative anomalies to the south. Negative rainfallanomalies first appear in the equatorial Indian Ocean atlag �2, then expand and propagate both eastward along theequator and northward in the Indian Ocean to the SCSsector.[30] Figure 9 shows Hovmoller diagrams of regressed

rainfall anomalies to further illustrate these propagationcharacteristics of convection. In the longitude-time sectionalong the equator, the eastward propagation is seen from60�E to 160�E, with some disruption over the MaritimeContinent. In the latitude-time section averaged from theBay of Bengal (80�E) to the SCS (120�E), convectiveanomalies first appear on the equator and then propagatenorthward onto the Asian Continent. The average eastwardand northward propagation speeds are 4�/day and 0.7�/day,respectively. The former (�5 m/s) is typical of atmosphericISV phase speed. The typical period of the convectiveoscillation is 6–7 weeks (�45 days).[31] The tilted, zonally elongated anomalous rain band,

the eastward propagation along the equator, and the north-ward propagation in the Bay of Bengal to the SCS sector areall known characteristics of the so-called boreal summerintraseasonal oscillation over South Asia and Indo-westernPacific warm pool region [e.g., Annamalai and Slingo,2001; Waliser et al., 2004; Jiang et al., 2004] (see Lauand Waliser [2005] and Wang [2006] for recent reviews). Infact, convective ISV displays a local variance maximum inthe SCS, and Straub and Kiladis [2003] use the time seriesin this region of maximum as the index for their analysis ofplanetary-scale ISV. Thus intraseasonal wind variabilityover the summer SCS is not a local phenomenon but partof planetary-scale organization of atmospheric convectionand circulation.

5. Ocean-to-Atmospheric Feedback

[32] To first order, the intraseasonal cooling of the SCSmay be considered as the oceanic response to planetary-scale atmospheric ISV but the cold filaments exert a visibleeffect onto surface wind. For this, we return to the intra-seasonal event that lasts for four weeks from late July toearly August 1999. During 28 July to 4 August, strongwinds above 10 m/s are observed along the wind jet axisthat tilts in the northeast direction (Figure 1). The fullydeveloped cold filament disrupts the high wind speed alongthis axis; during the week of 28 July, wind speed drops by2–3 m/s to below 10 m/s over the cold filament comparedto the values above 12 m/s both up and down wind. This

Figure 8. Lag-regression of rainfall (mm/day, colorshading) and 850 hPa wind (>0.05 m/s; in vector) uponthe ERAwind index during 10 summers (JJA) from 1993 to2002. Time lags are in week.


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wind speed minimum persists in the following week overthe cold filament, weakening but still visible for anotherweek.[33] The collocation and coevolution of the cold filament

and wind speed minimum are commonly observed in thesummer SCS as illustrated by the distance-time sectionsalong the northeast tilted wind jet axis (Figure 10). Thisaxis, taken based on the climatological wind distribution(Figure 4a), intercepts the southward curving part of thefilament arc. As another example, a wind speed reduction of3 m/s is also observed over the cold filament event duringlate August to early September 2000 (Figure 10b).[34] The wind response is a negative feedback for the

cold filament development: the local wind speed reductionand increased atmospheric stability act to suppress the latentheat flux from the ocean, damping the SST minimum. Sucha local wind response to SST variability is commonlyobserved near major ocean fronts in the world ocean [Xie,2004; Chelton et al., 2004], including the western ArabianSea off Somali [Vecchi et al., 2004], the East China Sea [Xieet al., 2002], and the Kuroshio Extension [Nonaka and Xie,2003]. Lin et al. [2003] observe such a wind speedreduction over the summer SCS over the cold wake leftbehind by a typhoon. Such ocean-to-atmospheric feedbackis characterized by a positive correlation between SST andwind speed anomalies and considered to be due to the SSTmodulation of near-surface atmospheric variability [Xie,2004]. Over the cold filament, the increased atmosphericstability reduces the surface wind stress and hampers thevertical mixing that would bring faster winds fromaloft. Recently, however, Small et al. [2005] suggest thatthe SST-induced pressure anomalies in the boundary layerexplain the positive correlation between SST and windspeed if the down-wind advection is considered.[35] Xie et al. [2003] draw an analogy between the

western Arabian Sea and SCS during summer in theirsharing common features such as an orographic-inducedsouthwest wind jet, a quasi-stationary anticyclonic eddysouth of the wind jet, strong coastal upwelling, the offshoredevelopment of cold filaments. This analogy becomes even

closer in the local wind response to the cold filamentdevelopment in both regions [Vecchi et al., 2004].

6. Summary

[36] We have studied intraseasonal variability in thesummer SCS using a suite of newly available, cloud-penetrating microwave observations from satellite. The highspatial resolution of these satellite data enables us to studymesoscale features such as the cold filaments and recircu-lation eddies in the ocean, and the narrow wind jet in theatmosphere. Our analysis shows that the SCS cold filamentdoes not develop smoothly on the seasonal timescale asdepicted in the climatological analysis of Xie et al. [2003],but instead it experiences pronounced intraseasonal cyclesof development and decay during a typical summer inresponse to the fluctuations of the southwest wind jet.The typical period of such cycles is 6–7 weeks, comparablewith the timescales of atmospheric ISV in the region [e.g.,Straub and Kiladis, 2003; Jiang et al., 2004]. The intra-seasonal cold filament development is subdued or suppressedcompletely in the summer following an El Nino event as thesouthwest monsoon weakens.[37] Our results suggest the following conceptual model

for summer ISV over the SCS. Intraseasonal wind variabil-ity in the summer SCS is part of a planetary-scale mode ofISV over South Asia and the Indo-Pacific warm pool,characterized by a tilted band of anomalous rainfall thatpropagates eastward along the equator and northward in theBay of Bengal to the SCS sector. Following the intensifi-cation of atmospheric convection over the central SCS aspart of this planetary-scale oscillation, the southwesterliesover the SCS strengthen, causing strong coastal upwellingoff South Vietnam in the subsequent week. Thiscold upwelled water is then advected offshore, forming anarc-shaped filament in SST. The offshore advection isprovided by an eastward ocean jet on the boundary of adouble gyre circulation forced by the wind curls of thesouthwest wind jet. The offshore cold filament leaves avisible imprint on the atmosphere, reducing the local windspeed by 2–3 m/s at the sea surface.

Figure 9. Lag-regression of rainfall anomaly (mm day�1) upon the ISV wind index from week�5 to +5:(a) equatorial (5�S–5�N) time-longitude diagram, and (b) latitude-time diagram in the Bay of Bengal toSouth China Sea sector (80�E–120�E).


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[38] Large SSH anomalies of 3–4 cm are observed overthe SCS. The strengthening of the double gyre circulationlags behind the wind intensification by 2–3 weeks, a delaydue to the westward propagation of ocean Rossby waves.The SSH variability is greater in the northern than thesouthern gyre because of the asymmetry in wind forcing;the blockage of the southwesterlies by the Annam mountainrange is strong in the lee, creating larger wind curls and astronger SSH response north than south of the axis of thesoutherly wind acceleration. Thus our composite analysiskeyed on a wind index reveals a SCS circulation adjustmentconsistent with the wind-forced Rossby wave mechanism.By construction, our method finds no evidence for internalvariability of the double gyre circulation playing a role inthe intraseasonal cold filament development. Instead, thewind-induced variability in the coastal upwelling is themain cause of the intraseasonal development and retreatof the cold filament. In a related study of another intergyreinertial jet, Taguchi et al. [2007] reach a similar conclusionthat the majority of decadal variability in the KuroshioExtension jet east of Japan is due to wind-forced baroclinic

Rossby waves despite considerable internal variability.They suggest that low-frequency wind forcing may regulatethe inertial jet’s internal variability.[39] Does the intraseasonal cooling of the SCS have any

influence on large-scale atmospheric convection and circu-lation? Our results tentatively suggest a positive answer. Inour composite analysis, the SST cooling (Figure 5c) andsuppressed atmospheric convection (Figure 8d) both occurat lag 1 week, suggesting that the former possibly forces thelatter. Figure 11 confirms that SST leads precipitation byone week over the central SCS on intraseasonal timescales.Similar covariations of SST and convection on intraseasonaltimescales have been noted by Sengupta et al. [2001] andVecchi and Harrison [2002] over the Bay of Bengal.Furthermore, coupled model experiments of Fu et al.[2003] show that the northward propagating ISV weakensin the atmosphere if the interaction with the ocean issuppressed. Fu et al. [2003] suggest that the wind-evaporation-SST (WES) feedback is the major mechanismfor ocean-atmosphere interaction over the Bay of Bengalon intraseasonal timescales. In fact, the WES mode of air-sea interaction under the prevailing westerly windsdisplays a poleward phase propagation [Xie, 1999], con-sistent with observations and Fu et al.’s model experi-ments. In the summer SCS, besides surface evaporation,the coastal upwelling and its offshore advection areimportant mechanisms for SST variability, possiblystrengthening ocean-atmosphere interaction there com-pared to the Bay of Bengal.[40] Intraseasonal variability is likely not to be limited to

physical fields. As the offshore advection carries from the

Figure 10. Time-distance diagrams of wind speed (ms�1,contour) and SST (�C, color shading) along the meansouthwest wind axis as marked in Figure 4a, during twointraseasonal wind events in (a) 1999 and (b) 2000.

Figure 11. Lag correlation (from lag �5 to +5 week) ofCMAP precipitation with the reference time series ofTRMM SST for JJA 1998–2004, both averaged over112.5–117.5�E, 10–15�N. Precipitation leads SST atnegative lags. With a typical ISV period of 45 days, weestimate the degree of freedom for the seven-summer timeseries to be 28, for which the correlation is 95% significantat 0.38 based on student-t test.


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Vietnam coast not only the cold upwelled water but alsohigh nutrients [Xie et al., 2003; Tang et al., 2004], it isconceivable that intraseasonal variability in coastal upwell-ing (the source region for the cold filament) causes fluctua-tions in offshore nutrients and biological activity over thecold filament. Recently it came to our attention thatIsoguchi and Kawamura [2006] has detected such intra-seasonal variability in chlorophyll from satellite. As windvariations are now being routinely monitored by satellite,the lags of 1–3 weeks in oceanic response offer usefulpredictability that may be exploited.

[41] Acknowledgments. We wish to thank B. Taguchi for technicalassistance, J. Hafner for maintaining a local satellite data archive, andP. Hacker for encouragement and support. The TMI and QuikSCAT data areobtained from the Web site of Remote Sensing Systems. This work waspresented at the South China Sea International Workshop in Guilin, China,July 2005 and the AGU Ocean Sciences Meeting, Honolulu, February2006. This work is supported by NASA, NOAA, the Japan Agency forMarine-Earth Science and Technology, the Chinese Academy of Sciences,and NSFC (40476013 and 40506008). IPRC publication 470 and SOESTpublication 7166.

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��C.-H. Chang and S.-P. Xie, International Pacific Research Center and

Department of Meteorology, University of Hawaii, HI 96822, USA.([email protected])D. Wang and Q. Xie, Laboratory for Tropical Marine Environmental

Dynamics, South China Sea Institute of Oceanology, Chinese Academy ofSciences, Guangzhou 510301, China. ([email protected]; [email protected])


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intraseasonal variability in the summer south china sea...

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